1. Field of the Invention
The invention relates to a semiconductor laser comprised of a pnpn thyrister and including a current strangulation structure, and also to a method of fabricating the same.
2. Description of the Related Art
A semiconductor laser including an active layer formed by selective growth and having a pnpn thyrister block structure formed by selective growth can be fabricated without carrying out a step of etching a semiconductor layer. Hence, a width of an active layer can be accurately controlled, ensuring uniformity in characteristic and reproducibility of a semiconductor laser.
FIG. 1 illustrates one of conventional semiconductor lasers having such a structure as mentioned above.
The illustrated semiconductor laser is comprised of an n-InP substrate 701, an electrode 711 formed on a lower surface of the n-InP substrate 701, a stripe including an n-InP clad layer 703, an MQW active layer 704, and a p-InP clad layer 705, a p-InP block layer 707 covering the n-InP substrate 701 and the stripe therewith, an n-InP block layer 708 formed on the p-InP block layer 707, a p-InP clad layer 709 formed on the n-InP block layer 708, a p-InGaAs cap layer 710 formed on the p-InP clad layer 709, and an electrode 712 formed on the p-InGaAs cap layer 710.
In the illustrated semiconductor laser, since a current is strangulated in the stripe including the MQW active layer 704, around the stripe is formed a pnpn thyrister block structure comprised of the n-InP substrate 701, the p-InP block layer 707, the n-InP block layer 708, and the p-InP clad layer 709. The pnpn thyrister block structure prevents a current from running outside the stripe.
Japanese Unexamined Patent Publication No. 5-67849 has suggested a semiconductor light-emitting device including a p-InP substrate formed with a mesa-stripe, an n-InP block layer, a p-InP buffer layer, an InGaAsP active layer, and n-InP clad layer all deposited on the p-InP substrate, a p-InP buried layer, an n-InP current-blocking layer, and a p-InP current-blocking layer all deposited in a recess formed in the InGaAsP active layer and the n-InP clad layer, an n-InP buried layer covering the n-InP clad layer and the p-InP current-blocking layer therewith, and a pair of electrodes.
Japanese Unexamined Patent Publication No. 8-330676 has suggested a semiconductor laser including a p-InP substrate, a pair of SiO2 stripe masks formed on the p-InP substrate in a [011] direction and spaced away from each other by 1.5 xcexcm, and a multi-layered structure including an active layer, formed in the 1.5 xcexcm-space by MOVPE selective growth.
Japanese Unexamined Patent Publication No. 9-266349 has suggested a semiconductor laser including a p-InP substrate including a buffer layer, a trapezoidal selective growth portion formed on the p-InP substrate. The selective growth portion includes a p-clad layer, SCH strain MQW layer, and n-clad layer, and acts as a waveguide for a laser having a wavelength of 1.3 xcexcm. Around the selective growth portion are deposited p-InP buried layer, n-InP layer, p-InP layer, and SCH-MQW carrier recombination layer. An n-InP clad buried layer covers the above-mentioned structure, and an n-InGaAsP contact layer is formed on the n-InP clad buried layer.
However, the above-mentioned conventional semiconductor lasers are accompanied with a problem that turn-on occurs in the pnpn thyrister at a high temperature or when much current is applied to the semiconductor lasers, and hence, it is not always ensured to provide a sufficient block breakdown voltage.
It is most effective to design a block layer to have a greater thickness in order to enhance a breakdown voltage of a thyrister. However, a thickness of a current-blocking layer has upper limitation in the above-mentioned conventional semiconductor lasers, because of a demand in a waveguide layer to have a small height. As a result, it is quite difficult or almost impossible in the above-mentioned conventional semiconductor lasers to make a thickness of a current-blocking layer greater in order to enhance a breakdown voltage of a thyrister.
In view of the above-mentioned problem, it is an object of the present invention to provide a semiconductor laser and a method of fabricating the same both of which is capable of preventing occurrence of turn-on in a pnpn thyrister, and providing a sufficient block breakdown voltage even at a high temperature or even when much current is applied to a pnpn thyrister.
In one aspect of the present invention, there is provided a semiconductor laser including (a) an n-type semiconductor substrate, (b) an active layer formed on the n-type semiconductor substrate, (c) a first p-type semiconductor layer formed adjacent to the active layer, (d) an n-type semiconductor layer formed adjacent to the first p-type semiconductor layer, (e) a second p-type semiconductor layer formed adjacent to the n-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer.
In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer suppresses electron-ejection into the first p-type semiconductor layer. As a result, it is possible to reduce the number of electrons passing through the first p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.
The lightly doped n-type semiconductor layer contains an impurity at preferably 3xc3x971017 cmxe2x88x923 or smaller, and more preferably at 1xc3x971017 cmxe2x88x923 or smaller. Such concentration of an impurity more effectively suppresses electron-ejection into the adjacent p-type semiconductor layer, ensuring an increase in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to a thyrister.
There is no lower limitation in the concentration of an impurity. However, it is preferable that the lightly doped n-type semiconductor layer contains an impurity at such a concentration that there does not occur auto-dope caused by a p-type impurity. For instance, the lightly doped n-type semiconductor layer contains an impurity at preferably 1xc3x971015 cmxe2x88x923 or greater.
The lightly doped n-type semiconductor layer has a thickness of preferably 0.5 xcexcm or greater, and more preferably 1.0 xcexcm or greater.
If the lightly doped n-type semiconductor layer is too thin, there might occur the tunneling effect in which electrons pass through the lightly doped n-type semiconductor layer. There is no upper limitation in a thickness of the lightly doped n-type semiconductor layer, unless the thickness does not deteriorate a structure of a semiconductor laser.
For instance, the lightly doped n-type semiconductor layer is designed to contain Si, S or Se as an n-type impurity.
It is preferable that the lightly doped n-type semiconductor layer extends entirely over p-n junction plane, which would effectively suppress electron-ejection into the adjacent p-type semiconductor layer.
For instance, the lightly doped n-type semiconductor layer may be formed in the n-type semiconductor substrate, in which case, it is preferable that the lightly doped n-type semiconductor layer has a depth equal to a thickness of the n-type semiconductor substrate.
For instance, the lightly doped n-type semiconductor layer may be formed on the n-type semiconductor substrate.
There is further provided a semiconductor laser including (a) a p-type semiconductor substrate, (b) an active layer formed on the p-type semiconductor substrate, (c) a first n-type semiconductor layer formed adjacent to the active layer, (d) a p-type semiconductor layer formed adjacent to the first n-type semiconductor layer, (e) a second n-type semiconductor layer formed adjacent to the p-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the p-type semiconductor layer and the second n-type semiconductor layer.
In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer formed between the p-type semiconductor layer and the second n-type semiconductor layer suppresses electron-ejection into the p-type semiconductor layer. As a result, it is possible to reduce the number of electrons passing through the p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.
The lightly doped n-type semiconductor layer contains an impurity at preferably 3xc3x971017 cmxe2x88x923 or smaller, and more preferably 1xc3x971017 cmxe2x88x923 or smaller.
For instance, the lightly doped n-type semiconductor layer may be designed to contain an impurity at 1xc3x971015 cmxe2x88x923 or greater.
There is still further provided a semiconductor laser including (a) a pnpn thyrister, (b) an n-type electrode making electrical contact with an n-type region located at an end of the pnpn thyrister, and (c) a lightly doped n-type semiconductor layer located adjacent to p-n junction plane closer to the n-type electrode among p-n junction planes of the pnpn thyrister.
In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer located adjacent to p-n junction plane closer to the n-type electrode among p-n junction planes of the pnpn thyrister suppresses electron-ejection into a p-type semiconductor region. As a result, it is possible to reduce the number of electrons passing through the p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.
In another aspect of the present invention, there is provided a method of fabricating a semiconductor laser, including the steps of (a) forming a lightly doped n-type semiconductor layer on or in an n-type semiconductor substrate, (b) ion-implanting an n-type impurity to the lightly doped n-type semiconductor layer to thereby partially turn the lightly doped n-type semiconductor layer into a heavily doped n-type semiconductor layer, (c) forming a light-waveguide layer on the heavily doped n-type semiconductor layer, the light-waveguide layer including an active layer, (d) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (e) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (f) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.
In accordance with the above-mentioned method, it is possible to fabricate a semiconductor laser having a high breakdown voltage, in the reduced number of fabrication steps at a high fabrication yield.
The heavily doped n-type semiconductor layer contains an impurity at preferably 5xc3x971017 to 5xc3x971018 cmxe2x88x923, and more preferably 1xc3x971018 to 3xc3x971018 cmxe2x88x923.
The lightly doped n-type semiconductor layer contains an impurity at preferably 3xc3x971017 cmxe2x88x923 or smaller, more preferably 1xc3x971017 cmxe2x88x923 or smaller, and preferably 1xc3x971015 cmxe2x88x923 or greater.
There is further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a heavily doped n-type semiconductor layer on or in an n-type semiconductor substrate, (b) forming a lightly doped n-type semiconductor layer entirely covering the heavily doped n-type semiconductor layer therewith, (c) at least partially removing the heavily doped n-type semiconductor layer and the lightly doped n-type semiconductor layer to thereby cause the heavily doped n-type semiconductor layer to appear, (d) forming a light-waveguide layer on the heavily doped n-type semiconductor layer, the light-waveguide layer including an active layer, (e) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (f) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (g) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.
The above-mentioned method makes it possible to fabricate a lightly doped n-type semiconductor layer including fewer defects such as crystal defect.
For instance, the heavily doped n-type semiconductor layer and the lightly doped n-type semiconductor layer may be partially removed in the step (c) by chemical mechanical polishing or dry etching.
There is still further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a lightly doped n-type semiconductor layer on an n-type semiconductor substrate, (b) patterning the lightly doped n-type semiconductor layer so that a space is formed in the thus patterned lightly doped n-type semiconductor layer, (c) forming a light-waveguide layer on the n-type semiconductor substrate in the space, the light-waveguide layer including an active layer, (d) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (e) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (f) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.
There is yet further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a first lightly doped p-type semiconductor layer on or in a p-type semiconductor substrate, (b) forming a light-waveguide layer on the first lightly doped p-type semiconductor layer, the light-waveguide layer including an active layer, (d) forming a first n-type semiconductor layer on the first lightly doped p-type semiconductor layer, (e) forming a first p-type semiconductor layer on the first n-type semiconductor layer, (f) forming an etching stopper layer on the first p-type semiconductor layer, (g) forming a second n-type semiconductor layer on the light-waveguide layer and the etching stopper layer, (h) etching the second n-type semiconductor layer so that a portion of the second n-type semiconductor layer remains only above the light-waveguide layer, and (i) depositing a second lightly doped n-type semiconductor layer on the etching stopper layer around the portion of the second n-type semiconductor layer.
There is still yet further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a first lightly doped p-type semiconductor layer on or in a p-type semiconductor substrate, (b) forming a light-waveguide layer on the first lightly doped p-type semiconductor layer, the light-waveguide layer including an active layer, (c) forming a first n-type semiconductor layer on the first lightly doped p-type semiconductor layer, (d) forming a p-type semiconductor layer on the first n-type semiconductor layer, (e) forming a second n-type semiconductor layer on the light-waveguide layer and the p-type semiconductor layer, (f) forming a mask on the second n-type semiconductor layer, the mask having an opening located just above the light-waveguide layer, and (g) ion-implanting an n-type impurity into the second n-type semiconductor layer through the opening of the mask.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the above-mentioned present invention, since a lightly doped n-type semiconductor layer is formed at a p-n junction plane, it is possible to enhance a block breakdown voltage at a high temperature or when much current is applied to a semiconductor laser, increase a saturated output, and reduce a drive current at a high temperature, ensuring longer lifetime of a semiconductor laser.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.